An inherent characteristic of a refrigerant compressor, such as used in an automotive air conditioning system, is the generation of dynamic pressure fluctuations, or pulsations, due to the dynamics of the compression process and the interaction of the gaseous refrigerant flow between the cylinders and the compressor. These pressure pulsations have the undesirable effect of vibrating certain components in the automotive air conditioning system, as well as components in the vehicle structure, which results in objectionable noise and/or destructive forces when the compressor rpm causes vibration at the resonant frequency of the system thus causing resonance. Also, the vibrating components are prone to more rapid wear and premature failure.
Swash plate refrigerant compressors having double-ended pistons are typically formed with an odd number of compression chambers at the front and rear ends of the compressor. For example, swash plate compressors may consist of three or five compression chambers at each end of the compressor. By forming an odd number of compression chambers on each side of the swash plate, only one compression chamber will be at the top dead center of the exhaust stroke at any one moment. Accordingly, in a six compression chamber compressor, i.e., three compression chambers at each of the front and rear ends of the compressor, there will be six equally spaced pressure pulsations per revolution of the swash plate. Hence, with each continued sixty degrees of rotation, the swash plate will move another piston to complete an exhaust stroke.
In theory, this equal spacing of exhaust strokes is highly advantageous because the discharge fluid pressure pulsations will be perfectly equally spaced in time from one another. In practice, however, the exhausted refrigerant from the front compression chambers is required to travel a greater distance to the exit port of the compressor than the exhausted refrigerant from the rear compression chambers. Therefore, the exhausted refrigerant from the front compression chambers must flow through a more restrictive path to the exit port. The additional distance and more restrictive flow path required to be traversed by the front compression chambers causes a time lag in the otherwise synchronized alternating pressure pulsations. Hence, when the exhaust flows from the front and rear compression chambers are mixed upstream of the exit port, they will no longer be perfectly spaced in time from each other.
At certain compressor speeds, this time lag can be so great as to cause the front compression chamber pressure pulsations to shift into phase with the rear compression chamber pulsations, thus causing destructive pressure pulsations of double magnitude throughout the system. That is, in a six cylinder swash plate compressor where one pressure pulsation normally occurs every sixty degrees of swash plate rotation, the additional time lag imposed on the three front compression chambers at certain rpm will sufficiently delay the merging of exhausted refrigerant fluid from the front compression chambers with the discharged fluid from the rear compression chambers so that one pressure pulsation of double magnitude occurs every one hundred and twenty degrees of swash plate rotation. Hence, at certain compressor speeds, instead of six pressure pulsations of a given magnitude chronologically spaced every sixty degrees of swash plate rotation, there will be three pressure pulsations of twice the given magnitude chronologically spaced every one hundred and twenty degrees of swash plate rotation.
In order to overcome this inherent defect, the prior art has taught to centrally locate the exit port between the front and rear compression chambers. For example, as shown in the U.S. Pat. No. 3,904,320 to Kishi et al, issued Sept. 9, 1975, and U.S. Pat. No. 4,863,356 to Akeda et al, issued Sept. 4, 1989, the exit port can be disposed midway between the front and rear ends of the compressor. Discharge flow passages extending from the front and rear compression chambers have substantially equal flow restrictions so that the pressure pulsations in the discharged fluid are always mixed out of phase. Hence, according to the Akeda et al '356 and the Kishi et al '320 teachings, a swash plate compressor having six compression chambers will be assured to have six equally chronologically spaced pressure pulsations per revolution at the exit port.
Although the prior art teachings are helpful in reducing the problem of phase shift, or time lag, in the discharge pressure pulsations, they are still insufficient to effectively muffle, or attenuate, all of the destructive pressure pulsations.